the commonest cause of not seeing periods at the expected time in females of reproductive age as yourself is pregnancy
. Having sex and ejaculation outside is a very unsafe contraceptive
method. Before ejaculation, some good quantity of seminal fluid already get out into the woman and there are marked chances of getting pregnant. Pregnancy in this case is not a rare possibility.
I would advise you get a pregnancy test done, and use other barrier methods
to prevent getting pregnant next time like condoms or pills. Your primary care
physician could help you on the various options that exist and you make a choice.
For now, you got to do a pregnancy test for it could be that you are pregnant and meet your family physician for advice.
Thanks and best regards,